DEVELOPMENT — YELK DIVISION. 205 



The answer to these questions is to be sought in the 

 facts of individual and ancestral development. 



An animal not only is, hut becomes ; the craj'fish is the 

 product of an egg, in which not a single structure visible 

 in the adult animal exists : in that egg the different tissues 

 and organs make their appearance by a gradual process of 

 evolution ; and the study of this process can alone tell 

 us whether the unity of composition suggested by the 

 comparison of adult structures, is borne out bj' the facts 

 of their development in the individual or not. The 

 hypothesis that the body of the crajfish is made up of a 

 series of homologous somites and appendages, and that 

 all the tissues are composed of nucleated cells, might be 

 onlj' a permissible, because a useful, mode of colligating 

 the facts of anatomy. The investigation of the actual 

 manner in which the evolution of the body of the crayfish 

 has been effected, is the only means of ascertaining 

 whether it is anj-thiiig more. And, in this sense, deve- 

 lopment is the criterion of all morphological speculations. 



The first obvious change which takes place in an im- 

 2)regnated ovum is the breaking up of the yelk into 

 smaller portions, each of which is provided with a nucleus, 

 and is termed a hlastomere . In a general morphological 

 sense, a hlastomere is a nucleated cell, and differs from 

 an ordinary cell only in size, and in the usual, though by 

 no means invariable, abundance of granular contents ; 

 and blastomeres insensibly pass into ordinary' cells, as 



